making GetDPIScaleFactor virtual, adding macOS direct impl

include/wx/osx/window.h

@@ -121,6 +121,7 @@ public:
 #endif // wxUSE_HOTKEY
 
     virtual wxSize GetDPI() const wxOVERRIDE;
+    virtual double GetDPIScaleFactor() const wxOVERRIDE;
 
 #if wxUSE_DRAG_AND_DROP
     virtual void SetDropTarget( wxDropTarget *dropTarget ) wxOVERRIDE;

include/wx/window.h

@@ -537,7 +537,7 @@ public:
 
     // Return the ratio of the DPI used by this window to the standard DPI,
     // e.g. 1 for standard DPI screens and 2 for "200% scaling".
-    double GetDPIScaleFactor() const;
+    virtual double GetDPIScaleFactor() const;
 
     // return the size of the left/right and top/bottom borders in x and y
     // components of the result respectively

src/osx/window_osx.cpp

@@ -2629,6 +2629,12 @@ wxSize wxWindowMac::OSXMakeDPIFromScaleFactor(double scaleFactor)
 }
 
 wxSize wxWindowMac::GetDPI() const
+{
+    return OSXMakeDPIFromScaleFactor(GetDPIScaleFactor());
+}
+
+// on mac ContentScale and DPIScale are identical
+double wxWindowMac::GetDPIScaleFactor() const
 {
     double scaleFactor;
     if ( wxNonOwnedWindow* tlw = MacGetTopLevelWindow() )
@@ -2636,9 +2642,10 @@ wxSize wxWindowMac::GetDPI() const
     else
         scaleFactor = wxOSXGetMainScreenContentScaleFactor();
 
-    return OSXMakeDPIFromScaleFactor(scaleFactor);
+    return scaleFactor;
 }
 
+
 //
 // wxWidgetImpl
 //